--- /dev/null
+<?php
+
+namespace Drupal\FunctionalJavascriptTests\Ajax;
+
+use Drupal\FunctionalJavascriptTests\JavascriptTestBase;
+
+/**
+ * Tests the compatibility of the ajax.es6.js file.
+ *
+ * @group Ajax
+ */
+class BackwardCompatibilityTest extends JavascriptTestBase {
+
+ /**
+ * {@inheritdoc}
+ */
+ public static $modules = [
+ 'js_ajax_test',
+ ];
+
+ /**
+ * Ensures Drupal.Ajax.element_settings BC layer.
+ */
+ public function testAjaxBackwardCompatibility() {
+ $this->drupalGet('/js_ajax_test');
+ $this->click('#edit-test-button');
+
+ $this->assertSession()
+ ->waitForElement('css', '#js_ajax_test_form_element');
+ $elements = $this->cssSelect('#js_ajax_test_form_element');
+ $this->assertCount(1, $elements);
+ $json = $elements[0]->getText();
+ $data = json_decode($json, TRUE);
+ $this->assertEquals([
+ 'element_settings' => 'catbro',
+ 'elementSettings' => 'catbro',
+ ], $data);
+ }
+
+}